1. Field of the Invention
The invention relates to asynchronous transmission networks, notably networks of the so-called ATM (asynchronous transmission mode) type.
2. Description of the Prior Art
In these networks, digital information elements are transmitted discontinuously in the form of packets, also known as "information cells" in ATM terminology. This mode of transmission, instead of being a byte-by-byte transmission, consists in forming a packet of bits in the sender terminal equipment (the source of data) as and when the digital data elements are created, waiting for the number of information elements to be sufficient to fill up a complete information cell and sending this cell on the network.
The sending equipment thus produces information cells at a variable rate, depending on whether or not it uses the channel (for example, no cell is sent out during the instants of silence) or depending on the variable rate at which the digital information elements are produced (for example in the typical case of differential video signals, the data volume of which depends on the variable extent to which the image to be transmitted is a moving image).
The information cell that will go through the network includes, firstly, a header carrying an address or "virtual channel identifier" and, secondly, a useful field carrying the information to be transmitted.
The information cells coming from several transmitters are mingled in a continuous stream, possibly with interposed empty cells that travel along communications links with high bit-rates.
The different arteries of the network connect a plurality of nodes, each corresponding to a switching device that carries out the demultiplexing/multiplexing of the stream of cells as a function of the address contained in the header and of the resources available at the instant considered.
One of the specific features of an ATM type network relates to the fact that all that the switch does is to manage solely the header of each information cell, in determining the destination address from the header of the incoming cell and in computing a new header from this parameter, said new header corresponding to the (virtual) channel which will have been assigned to the link that is switched over downline from this node.
In particular, no check (intrinsic error check, acknowledgment protocol, etc.) is made on the transmitted data elements, the checks if any being transferred to the ends of the line, i.e. to the two installations exchanging the information elements which should therefore check the integrity and conformity of the information elements exchanged.
Most usually, however, the ATM network is connected to a device transmitting in a mode other than the ATM mode, either because the asynchronous network is connected to a terminal installation of another type or because it is in fact only a sub-unit of a bigger network including, for example, a standard type of synchronous switch to which the asynchronous network is connected.
One of the aims of the present invention, particularly in a case such as this where the ATM network has to be coupled to a pre-existing device or switch of another type, is to enable the transmission of signalling data that is sent out synchronously.
Indeed, until now, the signalling data generated and received by the digital terminals of installations in synchronous mode called "out-of-band channel-by-channel" mode are assembled, in accordance with the CCITT G704 recommendation, in a "multiframe" circulating on one of the time intervals transmitted in synchronous mode.
More precisely, again in accordance with the above-mentioned recommendation, each frame is divided into 31 temporal units, namely 30 temporal units assigned to the transmission of the data elements proper (associated with 30 different channels) and one temporal unit that has been arbitrarily assigned the 16th position in the frame, containing the signalling data associated with these 30 channels.
More precisely, the temporal unit assembling these signalling data elements, also called a "multiframe", contains a sequence of 30 quadruplets, each one of which corresponds to the signalling data element (16 possible values) associated with each of the 30 channels.
In this standard mode of transmission, the synchronous frames therefore convey simultaneously the information elements proper (at a typical rate of 64 kbits/s) and the signalling data (at 2 kbits/s). Since the passbands that correspond to these two series of respective information elements are distinct, the term "out-of-band signalling" is used.
The ATM transmission mode is entirely different since, as indicated further above, the ATM network does not analyze the content of the information elements and therefore makes no distinction between information elements proper and signalling data elements.
A possible and immediate approach would consist in generating an ATM cell at each quadruplet received and in conveying this cell in the desired direction.
However, this approach would be very cumbersome to implement since a complete cell would have to be generated and conveyed at each instant of recurrence of the information element, i.e. every 2 ms (the ATM cells have a fixed length: it is therefore necessary to generate a complete cell to transmit a single quadruplet).
Apart from the cumbersome nature of its implementation, an approach such as this would lead to the creation of a large stream of cells, creating a non-negligible degree of congestion in the various communications links of the ATM network.